Why are you asking for help on a Monoprice printer in an Ultimaker Cura forum?
Several forums for your printer exist - google "monoprice printer forum" to find people who are more likely to have an answer.

No, and top/bottom aren't infill, they are walls, so they get the same extrusion as other walls, although you can separately set the number of top and bottom layers.
Slowing down the speed should work for what you are asking, and for top layers, you might look at the Ironing settings for even better top quality.

Now perhaps you could turn your attention to writing something that would carry Machine settings forward from one version of Cura to the next?
It is infuriating to have to recreate all the Machine settings every time Cura drops a new version, which seems to be at least monthly lately. I can't believe that the Cura team keeps propagating this stupid "feature" after many years.

I must be missing something: using Moment of Inspiration (moi3d.com) I created a cube and a smaller sphere. I centered the sphere inside the cube and used Boolean Diff to hollow out the cube with the sphere.
Saved as STL file (attached), and opened it in Cura 15.04, where the Layers view showed clearly that the cube had a hollow spherical center hole.
So the answer is that Yes, you can indeed create custom supports inside an object by making inner holes and printing the object mostly solid otherwise.
You might also want to try the ability to use Concentric with a large value of Number of Walls, to make the "grain" of the printing follow the outline of your internal supports.
hollow.stl

a )If the wall 1.2mm thickness is important, consider using a .6mm nozzle. It will print twice as much plastic vs. a .4mm, and you can print the piece with two walls instead of three.
b) Or, try specifying a large number of walls, to force solid concentric-pattern printing with no infill.
c) Also, try varying the line width by tenths of a mm or so - both up and down - experiment with it, along with Equalize Filament Flow and Compensate Wall Overlaps, you should be able to get higher density.